This invention is a mechanism and associated circuitry which allows the use of a low-cost dc motor to be used in an electronic typewriter to drive the ribbon advance and ribbon lift functions, and more specifically, comprises a timing disk and microprocessor controlled motor driver to accurately control the dc motor.
In basic electronic typewriters, the lowest cost components must be used to drive the various moving parts which include the ribbon advance and ribbon lift. Solenoids and stepping motors in conjunction with various cam-driven mechanisms have been used because a stepping motor torque, speed and angular position are easily controlled, but a stepping motor and its drive circuit are a relatively expensive solution to the problem in a low-cost typewriter. Specifically designed dc motors with clutches and lift solenoids have also been used, but the cost of these components is relatively high. In all cases the goal is to use the lowest cost parts to achieve reliable operation.
Direct current motors such as are used in household hair dryers and can openers are manufactured in numbers exceeding a hundred million per year, and as a result of the continuing engineering applied to these motors, can be specified and purchased in a range of reliability, speed and power at very low cost. The drive circuit for a dc motor is also very simple since the motor has commutator rings that allow it to be driven from a simple dc source. The disadvantage of a motor of this type is that there is normally very little control over the speed and torque of the motor. In its normal use, a voltage is applied, and the motor will spin up to its terminal speed which is quite variable depending on voltage fluctuations, load and individual motor characteristics. This lack of speed control would be a large disadvantage in a typewriter where the operator needs a constant and predictable action from the typewriter in response to the key strokes to establish a comfortable rhythym.
What is required is a low cost circuit, motor and associated mechanism for controlling the ribbon advance and ribbon lift in an electronic typewriter.